Became a possibility when people realized that flipping through pages of related pictures produced the illusion of movement. each variation of the image on each page, is slightly different then the one before, which creates the illusion in our mind.
a. sound
c. film emulsion
b. film
d. panning shots

Film. 
This describes how things are animated even today

These traits are all equally important in recognizing the validity of artwork mainly due to an artists' consistent style and reputation which they have developed over a lengthy period of time. most famous paintings contain specific characteristics of the artist who created it. an appraiser would pay close attention to certain key elements within the painting, as well as an accuracy in measurement and color shade. blended textures and outlines are essential in recognizing an authentic piece from a copy. signature and imagery exist in epic proportions to balance out each trait as they coexist in harmony, depending on preference and style of each artist. no two original pieces are virtually identical, although it may be almost impossible to see discrepancy with the naked eye.